The embodiments disclosed herein relate to an apparatus and method for lifting and/or stabilizing a web, particularly applicable in intermittent coating operations, such as those used in manufacturing batteries, where the substrate is coated in a series of discrete patches. Further embodiments relate to a method for controlling said apparatus to provide precise control of length and thickness profile of said discrete coating patches.
There are various applications in which it is desirable to deposit a coating onto at least a portion of a sheet of material. For example, in some embodiments, the electrodes of batteries are produced by applying a layer or coating to a sheet, and then cutting the sheet into portions of a suitable dimension. Of particular importance is that the layer be applied at a uniform thickness. In some embodiments, the layer or coating is not applied to the sheet in the region where the sheet will subsequently be cut.
In the manufacture of lithium ion batteries and the like, there is a coating process that applies anode slurry to a conductive substrate (e.g., copper foil) and another coating process that applies cathode slurry to a conductive substrate (e.g., aluminum foil). In these two coating processes, there are two different methods of coating: discontinuous, also referred to as skip or patch coating, and continuous coating. In the practice of either method, the coating material may be applied to the continuously moving substrate in the form of one or more lanes running parallel to the travel direction of said continuously moving substrate. One method of coating known to those skilled in the art has a backing roller on which the moving substrate is conveyed in an arcuate path as it is supported and is positioned precisely by the surface of said backing roller. In some cases, it is not convenient or even possible to allow the web to contact a backing roller, such as in the case of coating both sides of the web with a wet material before both said applied coatings are dried. In the practice of the embodiments disclosed herein, the web is conveyed in a free span between web support elements. Said web support elements could be one or more idler rollers, vacuum tables or air flotation bars which position and guide the path of web travel in a straight path.
An example of such a prior art system is shown in FIG. 15, wherein slurry is applied to the moving substrate web 310 in a free span between web support elements 315 and 320 via a slot die coater 70 attached to a pumping station. The coating is typically held in a tank or reservoir 30. The coating is drawn from the reservoir 30, through conduit 31 by pump 40. The coating is then passed through conduit 32 by the action of the pump 40. In the case where coating is not being applied to the sheet 10, bypass valve 63 is open while supply valve 60 is closed. This allows the coating that is pumped through conduit 32 to pass through conduit 33 and back to reservoir 30. In the case where coating is being applied to the sheet 10, the bypass valve 63 is closed, while supply valve 60 is opened. This permits the flow of coating through conduit 62 to the nozzle 70, and onto the sheet 10. While the supply valve 60 is open, the coating is discharged by the nozzle 70. However, when the supply valve 60 is closed, the pressure needed to propel the coating through the nozzle 70 is eliminated. In some cases, this causes excess coating material to remain in the cavity, or manifold 71, and the lips 72 of the nozzle. When the supply valve is next opened, this excess material may cause an uneven application of coating to the sheet 10. FIG. 16 shows an example of the result of this phenomenon on the coated patch thickness. Coated patch 500 is shown as a cross-section profile of thickness “x” applied to web 10. As the sheet moves toward the left, starting profile 520 is thicker than the rest of the coating 500. This excess material 510 is due to the residual coating material that remained in the nozzle 70 after the supply valve 60 was closed. In this figure, the ending profile 525 is shown to be uneven, as the valves may be transitioning while the coating is still being applied. Such an uneven coating may be unacceptable.
Therefore, to prevent this uneven application, a fluid suction mechanism 80′ may be used, as shown in FIG. 15. This fluid suction mechanism is used to draw the excess coating that is left in the manifold 71 or on the lips 72 away from the nozzle 70. In operation, pump 40 draws coating material from reservoir 30. The coating material passes through conduits 31, 32 and is directed toward the nozzle 70, where it is discharged onto the sheet 10 as the sheet is drawn past roller 15. To stop the flow of coating onto the sheet 10, the bypass valve 63 is opened and the supply valve 60 is closed, thereby diverting the coating material through conduit 33 and back into the reservoir 30. To remove excessive coating material that may be present in the manifold 71 or on the lips 72 of the nozzle 70, valve 85 is opened to suction source 80 so that fluid is drawn by vacuum through conduit 86 which is in fluid communication with die manifold 71. The suction source 80 is typically comprised of a vacuum reservoir tank in communication with a suction pump to create a draw of fluid from die cavity 71 when valve 85 is opened. Coating fluid material is collected in said reservoir tank and periodically removed for reuse or, more often, discarded as waste material.
To restart the flow of coating onto the sheet 10, valve 85 is closed to remove the vacuum drawing fluid through conduit 86. Bypass valve 63 is closed while supply valve 60 is opened.
In the practice of free span coating, the planarity of the web is of significant importance in applying a uniform thickness of coating fluid to the web in the direction of web travel and in the cross-web direction. As the foil web approaches the slot die coater, the web must remain flat as it travels over the slot die coater, but due to a baggy web or tension corrugation in the thin foil, the web will tend to lift off the slot die coater or otherwise deviate from the desired path of travel resulting in a non-uniform gap between the fluid discharge lips of said slot die coater and the web surface to be coated. Without a uniform gap to the slot die coater discharge lips, the coating process creates defects in the coated web, such as non-uniform thickness of applied coating, ridges or streaks.
It therefore would be desirable to provide an apparatus and method for stabilizing the web in the free span to help provide defect-free coatings. It would also be desirable to utilize the same apparatus to move the web relative to the slot die coater to an off coat position in order to create the uncoated portion of the web, and return the web to an on coat position in order to create the coated portion of the web. This web movement would be especially useful in discontinuous coating of patches in precise position spacing and uniformity to precisely control the lengths and thickness profile of the coated and non-coated patches along the direction of travel. It would further be desirable to provide a device for guiding and flattening a running web.